1. Field of the Invention
The present invention relates to high frequency heating generators. These generators may be used notably in devices that achieve heating by electromagnetic induction, dielectric losses or plasma.
Generators such as these generally have a resonant circuit that is electromagnetically coupled to a part to be heated, connected to an output of at least one electron tube mounted as a self-oscillator.
2. Description of the Prior Art
Heating by electromagnetic induction consists in causing a conductive part to get heated by the circulation of currents induced by a magnetic field. This method enables the part to be heated in its mass without direct contact with the energy source. The part to be heated (or the part subjected to induction) is surrounded by at least one winding for the circulation of current (or inductor). The working frequencies of the generator range from some tens of kilohertz to some megahertz. The power values needed range from some kilowatts to more than one megawatt. Heating by electromagnetic induction is used extensively in industry and in the scientific field. In industry, it is used notably in metallurgy to refine metals, conduct heat processing operations on metal parts or produce continuously welded tubes.
Heating by dielectric losses consists in causing an insulator part to get heated by prompting losses in its mass, through an AC electrical field. The part to be heated is a poor insulator. It is placed between two conductive plates supplied by an AC source. A capacitor is created, the dielectric of which is the part to be heated. The generators used generally have higher working frequencies than those of heating generators that work by electromagnetic induction. These frequencies may range from some tens of megahertz to some gigahertz. This mode of heating is used in the wood industry for drying or bonding, in the textile industry or in the manufacture or shaping of plastic materials.
Heating by plasma consists in ionizing a gas medium to convert it into plasma. The kinetic, energy gets converted into heat. There is a considerable considerably rise in temperature. The part to be heated is placed in the plasma. The conversion of the gas medium into plasma is obtained by an emission from an antenna. The working frequencies of the generator range from one megahertz to some tens of megahertz. This mode of heating is used in numerous industrial applications such as the melting of refractory products, chemical synthesis etc.
The performance values of high frequency heating generators are optimal if the load impedance presented to the electron tube is mttched at all times. The resonant circuit electromagnetically coupled to the part to be heated is equivalent either to a parallel R,L,C circuit or to a series R,L,C circuit. The overvoltage or quality factor Q of the resonant circuit is high, and mismatching is easy in the vicinity of the resonance frequency. For the load impedance presented to the tube to be matched, it should be real, and its modulus should have an optimal value for the tube. The load impedance is essentially variable for it depends on the characteristics of the elements constituting the resonant circuit and notably of the material of the part to be heated, their dimensions and their relative position. This position is important especially if the part to be heated is in motion, for example if it is an induction heated plate of sheet metal that is rolled and continuously welded.
Most of the characteristics vary also with temperature and time. The variation of the impedance leads to a variation of the resonance frequency of the resonant circuit. The performance characteristics of electron tubes are affected by these variations. Their efficiency deteriorates. Since the tube is used in a self-oscillator assembly, its working frequency is influenced by the resonance frequency of the resonant circuit without being thereby identical in any way. The variation of the resonance frequency may lead to a stalling of the oscillator which then ceases to oscillate. All these cases of mismatching necessitate an over-excitation of the tube which may damage it.
To match the load impedance presented to the tube, it is possible to insert commutable elements in a matching transformer interposed between the oscillator tube and the resonant circuit This matching is done before the start of the heating as a function of the assumed impedance that the load will have.
It is also possible to interpose a circuit comprising variable inductances and capacitances between the resonant circuit and the oscillator tube. These variable elements are permanently servo-linked so that the resonance frequency of the circuit is tuned to the chosen working frequency of the generator and so that the modulus of the impedance permanently keeps its optimum value. The servo-linking mechanisms are often electromechanical. This system is used when high power values are brought into play. The elements used are expensive for they are sized so as to receive high currents. They are bulky and introduce Joule effect losses that are not negligible.